Succeeded in handing off the servo from the green to the red.
This time we found that the fluctuation in the IR signals became lesser as the gain of the ALS servo increased.
Therefore I increased the UGF from 40 Hz to 180 Hz to have less noise in the IR PDH signal.
Here is a preliminary plot for today's noise spectra.
The blue curve is the ALS in-loop spectrum, that corresponds to the beat fluctuation.
The red curve is an out-of-loop spectrum taken by measuring the IR PDH signal.
Since the UGF is at about 180 Hz the rms is integrated from 200 Hz.
The residual displacement noise in the IR PDH signal is now 1.2 kHz in rms.
I am going to analyze this residual noise by comparing with the differential noise that I took yesterday (see the last entry ).
With the exception of a 2" mirror mount, I've confirmed that we have everything for the Y-end green production and mode-matching.
We need to calculate a mode-matching solution for the Lightwave laser so that it gives the correct beam size in the doubling crystal.
Additionally, Rana has suggested that we change the pedestals from the normal 1" diameter pedestal+fork combo to the 3/4" diameter posts and wider bases that are used on the PSL table (as shown in the attached image).
There was a 2" mirror mount among the spares on the PSL table. It has a window LW-3-2050 UV mounted in it. I
have moved it to the Y-end table. We seem to have run out of 2" mirror mounts ...
Just a quick update... the Lightwave laser has now been moved up to the end of the Y arm. It's also been mounted on the new mounting block and heatsinks attached with indium as the heat transfer medium.
A couple of nice piccies...
The good news is that we seem to be running in a linear region of the PSL laser with a degree or so of range before the PSL Innolight laser starts to run multi-mode. On the attached graph we are currently running the PSL at 32.26degrees (measured) which puts us in the lower left corner of the plot. The blue data is the Lightwave set temperature (taken from the display on the laser controller) and the red data is the Lightwave laser crystal measured temperature (taken from the 10V/degC calibrated diagnostic output on the back of the laser controller - between pins 2 and 4).
The other good news is that we can see the transition between the PSL laser running in one mode and running in the next mode along. The transition region has no data points because the PMC has trouble locking on the multi-mode laser output - you can tell when this is happening because, as we approach the transition the PMC transmitted power starts to drop off and comes back up again once we're into the next mode region (top left portion of the plot).
The fitted lines for the region we're operating in are:
Y_arm_Temp_meas = 0.95152*T_PSL + 3.8672
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
X_arm and Y_arm vs PSL comparison.
Just a quick check of the performance of the X arm and Y arm lasers in comparison to the PSL. Plotting the data from the X arm vs PSL and Y arm vs PSL on the same plot shows that the X arm vs the PSL has no observable trending of mode-hopping in the laser, while the Y arm vs the PSL does. Suspect this is due to the fact that the X arm and PSL are both Innolight lasers with essentially identical geometry and crystals and they'll tend to mode-hop at roughly the same temperatures - note that the Xarm data is rough grained resolution so it's likely that any mode-hop transitions have been skipped over. The Lightwave on the other hand is a very different beast and has a different response, so won't hop modes at the same temperatures.
Given how close the PSL is to one of the mode-transition regions where it's currently operating (32.26 degC) it might be worth considering shifting the operating temperature down one degree or so to around 31 degC? Just to give a bit more headroom. Certainly worth bearing in mind if problems are noticed in the future.
Kiwamu and I looked at all the electronics that are currently in place for the green locking on the X-arm and have made a set of block diagrams of the rack mounted units that we should build to replace the existing ... "works of art" that sprawl around out there at the moment.
1. "ETM Green Oscillator/PDH support box". Not a great name but this would provide the local oscillator signal for the end PDH (with a controllable phase rotator) as well as the drive oscillator for the end laser PZT. Since we need to hit a frequency of 216.075kHz with a precision that Kiwamu needs to determine, we'd need to be able to tune the oscillator ... it needs to be a VCO. It'd be nice to be able to measure the output frequency so I've suggested dividing it down by N times to put it into the DAQ - maybe N = 2^7 = 128x to give a measured frequency of around 1.7kHz. Additionally this unit will sum the PDH control signal into the oscillation. This box would support the Universal PDH box that is currently at the X-end.
2. "Vertex X-arm beatnote box" - this basically takes the RF and DC signals from the beatnote PD and amplifies them. It provides a monitor for the RF signal and then converts the RF signal into a square wave in the comparator.
3. "Mixer Frequency Discriminator" - just the standard MFD setup stored in a box. For temperature stability reasons, we want to be careful about where we store this box and what it is made of. That's also the reason that this stage is separated from the X-arm beatnote box with it's high-power amps.
4. RS232 and EPICS control of the doubling ovens
5. Intensity stabilization of the End Laser
P.S. I used Google Diagrams for the pictures.
Today we tried the Schmitt trigger DFD, and while it works it does not improve the noise performance. At least part of our problem is coming from the discrete nature of our DFD algorithm, so I would propose that an industrious day job person codes up a new DFD which avoids switching. We can probably do this by mixing the input signal (after high-passing) with a time-delayed copy of itself... as we do now, but without the comparator. This has the disadvantage of giving an amplitude dependent output, but since we are working in the digital land we can DIVIDE. If we mix the signal with itself (without delay) to get a rectified version, and low-pass it a little, we can use this for normalization. The net result should be something like:
output = LP2[ s(t) * s(t - dt) / LP1[ s(t) * s(t) ]],
where s(t) is the high-passed input and LP is a low-pass filter. Remember not to divide by zero.
I modified the c1gfd.mdl simulink model. I made a backup as c1gfd_20110325.mdl.
The first change was to use a top_names block to put everything in. The block is labeled ALS. So all the channels will now be C1:ALS-GFD_SOMETHING. This means medm channel names will need to be updated. Also, the filter modules need to be updated in foton because of this.
I then proceeded to add the suggested changes made by Matt. To avoid a divide by zero case, I added a saturation part which saturates at 1e-9 (note this is positive) and 1e9.
I measured some laser powers associated with the beat-note detection system on the PSL table.
The diagram below is a summary of the measurement. All the data were taken by the Newport power meter.
The reflection from the beat-note PD is indeed significant as we have seen.
In addition to it the BS has a funny R/T ratio maybe because we are using an unknown BS from the Drever cabinet. I will replace it by a right BS.
During my work for making a noise budget I noticed that we haven't carefully characterize the beat-note detection system.
The final goal of this work is to draw noise curves for all the possible noise sources in one plot.
To draw the shot noise as well as the PD dark noise in the plot, I started collecting the data associated with the beat-note detection system.
* Estimation and measurement of the shot noise
* measurement of the PD electrical noise (dark noise)
* modeling for the PD electrical noise
* measurement of the doubling efficiency
* measurement of an amplitude noise coupling in the frequency discriminators
In the last week Matt and I modified the MFD configuration because the mixer had been illegally used.
Since the output from the comparator is normally about 10 dBm, a 4-way power splitter reduced the power down to 4 dBm in each output port.
In order to reserve a 7 dBm signal to a level-7 mixer, we decided to use an asymmetric power splitter, which is just a combination of 2-way and 3-way splitter shown in the diagram above.
With this configuration we can reserve a 7 dBm signal for a mixer in the fine path.
However on the other hand we sacrificed the coarse path because the power going to the mixer is now 2.2 dBm in each port.
According to the data sheet for the mixer, 1 dB compression point for the RF input is 1dBm. Therefore we put a 1 dB attenuator for the RF port in the coarse system.
In the delay line of the fine path we found that the delay cable was quite lossy and it reduced the power from 2.2 dBm to about 0 dBm.
Using 2 dBm for a Level 7 mixer is so bogus, that I will dismantle this as soon as I come over.
PLEASE DO NOT DISMANTLE THE SETUP !
Actually we tried looking for a level-3 or a smaller mixer, but we didn't find them at that moment. That's why we kept the level-7 mixer for the coarse path.
As you pointed out we can try an RF amplifier for it.
Right. I've got a whole load of info and data and assorted musings I've been saving up and cogitating upon before dumping it into these hallowed e-pages. there's so much I'll probably turn it into a threaded entry rather than put everything in one massive page.
An overview of what's coming:
I started out using http://lhocds.ligo-wa.caltech.edu:8000/40m/Advanced_Techniques/Green_Locking?action=AttachFile&do=get&target=modematch_END.png as a reference for roughly what we want to achieve... and from http://nodus.ligo.caltech.edu:8080/40m/100730_093643/efficiency_waist_edit.png we need a waist of about 50um at the green oven. Everything else up to this point is pretty much negotiable and the only defining things that matter are getting the right waist at the doubling oven with enough available power and (after that point) having enough space on the bench to separate off the green beam and match it into the Y arm.
Step 1: Measure the properties of the beam out of the laser. Really just need this for reference later because we'll be using more easily measurable points on the bench.
Step 2: Insert a lens a few cm from the laser to produce a waist of about of a few 100um around the Faraday. Note that there's really quite a lot of freedom here as to where the FI has to be - on the X arm it's around columns 29/30 on the bench, but as long as we get something that works we can get it closer to the laser if we need to.
Step 3: After inserting the FI need to measure the beam after it (there *will* be some distortion and the beam is non-circular to begin with)
Step 3b: If beam is non-circular, make it circular.
Step 4: Insert a lens to produce a 50um waist at the doubling oven position. This is around holes 7/8 on the X arm but again, we're free to change the position of the oven if we find a better solution. The optical set-up is a little bit tight near that side of the bench on the X end so we might want to try aiming for something a bit closer to the middle of the bench? Depends how the lenses work out, but if it fits on the X end it will fit on the Y end.
RIght! Overview out of the way - now comes the trivial first bit
Step 1: Beam out of the laser - this will be tricky, but we'll see what we can actually measure in this set-up. Can't get the Beamscan head any closer to the laser and using a lambda/2 plate + polariser to control power until the Faraday isolator is in place. Using 1 inch separation holes as reference points for now - need better resolution later, but this is fine for now and gives an idea of where things need to go on the bench. The beam is aligned to the 3rd row up (T) for all measurements, the Beamscan spits out diameters (measuring only the 13.5% values) so convert as required to beam radius and the beam is checked to ensure a reasonable Gaussian profile throughout.
Position A1_13.5%_width A2_13.5%_width
(bench) (um mean) (um mean)
32 2166.1 1612.5
31 2283.4 1708.3
30 2416.1 1803.2
29 2547.5 1891.4
27 2860.1 2070.3
26 2930.2 2154.4
25 3074.4 2254.0
24 3207.0 2339.4
OK. As expected, this measurement is in the linear region of the beampath - i.e. not close to the waist position and beyond the Rayleigh length) so it pretty much looks like two straight lines. There's no easy way to get into the path closer to the laser, so reckon we'll just need to infer back from the waist after we get a lens in there. Attached the plot, but about all you really need to get from this is that the beam out of the laser is very astigmatic and that the vertical axis expands faster than the horizontal.
Not terribly exciting, but have to start somewhere.
Step 2: Getting the beam through the Faraday isolator (FI).
Started out with an f=100mm lens at position 32,T on the bench which gave a decent looking waist of order 100 um in the right sort of position for the FI, but after checking the FI specs, it's limited to 500W/cm^2. In other words, if we have full power from the laser passing into it we'd need a beam width of more than 211 um. Solution? Use an f=150mm lens instead and don't put the FI at the waist. I normally don't put a FI at a waist anyway, for assorted reasons - scattering, thermal lensing, non-linear magnetic fields, the sharp changing of the field components in an area where you want as constant a beam as possible. Checked with others to make sure they don't do things differently around these parts… Koji says it doesn't matter as long as it passes cleanly through the aperture. So… next step is inserting the Faraday.
The beam profiles in vertical and horizontal around the FI position with the f=150mm lens in place are attached. Note that the FI will be going in at around 0.56m.
I fired up some old waistplotter routines, and set the input conditions as the measured waist after the lens and used that to work out what the input waist is at the laser. It may not be entirely accurate, but it /will/ be self consistent later on.
Vertical waist = 105.00 um at 6.282 cm after laser output (approx)
Horizontal waist = 144.63 um at 5.842 cm after laser output (approx)
Step 3: Inserting FI and un-eliptical-ification of the beam
The FI set up on it's mount and the beam passes through it - centrally through the apertures on each side. Need to make sure it doesn't clip and also make sure we get 93% through (datasheet specs say this is what we should achieve). We will not achieve this, but anything close should be acceptable.
Setting up for minimum power through the FI is HWP @125deg.
Max is therefore @ 80deg
Power before FI = 544 mW
Power after FI = 496 mW (after optimising input polarisation)
Power dumped at input crystal = 8.6mW
Power dumped at input crystal from internal reflections etc = 3.5mW
Power dumped at output crystal on 1st pass = approx 8mW
OK. that gives us a 90.625% transmission and a 20.1mW absorption/unexplained loss.
Well - OK. The important part about isolators isn't their transmission, it's about how well they isolate. Let's see how much power gets ejected on returning through the isolator…
Using a beam splitter to pick off light going into and returning from the FI. A 50/50 BS1-1064-50-1025-45P. And using a mirror near the waist after the FI to send the beam back through. There are better ways to test the isolation performance of FI's but this will suffice for now - really only want to know if there's any reasonable isolation at all or if all of the beam is passing backwards through the device.
Power before BS = 536 mW (hmmn - it's gone down a bit)
Power through BS = (can't access ejected on first pass)
Power through FI = 164 mW (BS at odd angle to minimise refractive effect so less power gets through)
Power lost through mirror = 8.3mW (mirror is at normal incidence so a bit transmissive)
Using earlier 90.6% measurement as reference, power into FI = 170.83 mW
So BS transmission = 170.83/536 = 0.3187
BS reflectivity therefore = 1 - 0.3187 = 0.6813
Power back into FI = Thru FI - Thru mirror = 155.7 mW
Power reflected at BS after returning through FI = 2.2mW
Baseline power at BS reflection from assorted internal reflections in FI (blocked return beam) = 1.9mW
Note - these reflections don't appear to be back along the input beam, but they *are* detectable on the power meter.
Actual power returning into FI that gets reflected by BS = 0.3 mW
(note that this is in the fluctuating noise level of measurement so treat as an upper limit)
Accounting for BS reflectivity at this angle, this gives a return power = 0.3/0.6813 = 0.4403 mW
Reduction ratio (extinction ratio) of FI = 0.4403/155.7 = 0.00282
Again - note that this upper limit measurement is as rough and ready as it gets. It's easy to optimise this sort of thing later, preferable on a nice open bench with plenty of space and a well-calibrated photodiode. It's just to give an idea that the isolator is actually isolating at all and not spewing light back into the NPRO.
Next up… checking the mode-matching again now that the FI is in place. The beam profile was scanned after the FI and the vertical and horizontal waists are different...
Step 3b: Non-circular? We can fix that...
A quick Beamscan sweep of the beam after the Faraday:
25.8 503.9 478.8
25 477.5 489.0
24 447.1 512.4
21 441.6 604.5
20 476.3 645.4
19 545.4 704.1
18 620.3 762.8
OK. It looks not too bad - doesn't look too different from what we had. Note that the x axis is in local table units - I found this useful for working out where things were relative to other things (like lenses and the FI) - but it means the beam propagates from right to left in the plot. in other words, the horizontal waist occurs first and is larger than the vertical waist. Also - they're not fitted curves - they're by-eye, best guesses and there's no solution for the vertical that doesn't involve offsets... discussion in a later part of the thread.
Anyway! The wonderful thing about this plot is that the horizontal and vertical widths cross and the horizontal focussing at this crossing point is shallower than the vertical. This means that we can put a lens in at the crossing point and rotate it such that the lens is stronger in the horizontal plane. The lens can be rotated until the effective horizontal focal length is right to fix the astigmatism.
I used a 200mm lens I had handy - a rough check sweeping the Beamscan quickly indicated should be about right though. Adjusting the angle until the beam size at a distant point is approx circular - I then move the profiler and adjust again. Repeat as required. Now… taking some data. with just that lens in:
24 371.7 366.1
21 360.3 342.7
20 447.8 427.8
19 552.4 519.0
18 656.4 599.2
17 780.1 709.9
16 885.9 831.1
Well now. That looks quite OK. Fit's a bit rubbish on vertical but looks like a slight offset on the measurement again.
The angle of the lens looks awful, but if it's stupid and it works then it isn't stupid. If necessary, the lens can be tweaked a bit more, but there's always more tweaking possible further down the line and most of the astigmatic behaviour has been removed. It's now just a case of finding a lens that works to give us a 50 um beam at the oven position...
Step 4: Matching into the oven
Now that the astigmatism is substantially reduced, we can work out a lens solution to obtain a 50um waist *anywhere* on the bench as long as there's enough room to work with the beam afterwards. The waist after the Faraday and lens is at position 22.5 on the bench. A 50 mm lens placed 18 cm after this position (position 14.92 on the bench) should give a waist of 50 um at 24.57 cm after the waist (position 12.83 on the bench). This doesn't give much room to measure the beam waist in though - the Beamscan head has a fairly large finite size… wonder if there's a slightly less strong lens I could use…
OK. With a 66 mm lens at 23 cm (position 13.45 on the bench) after the waist we get a 50 um waist at 31.37 cm after the waist (position 10.15 on the bench).
Closest lens I found was 62.9mm which will put the 50um point a bit further towards the wall, but on the X-arm the oven is at position 8.75 ish. So anything around there is fine.
Using this lens and after a bit of manual fiddling and checking with the Beamscan, I figured we needed a close in, fine-grained measurement so set the Beamscan head up on a micrometer stage Took a whoie bunch of data around position 9 on the bench:
(mm) (um mean) (um mean)
-15 226.8 221.9
-14 210.9 208.3
-13 195.5 196.7
-12 181.0 183.2
-11 166.0 168.4
-10 154.0 153.1
-9 139.5 141.0
-8 127.5 130.0
-7 118.0 121.7
-6 110.2 111.6
-5 105.0 104.8
-4 103.1 103.0
-3 105.2 104.7
-2 110.9 110.8
-1 116.8 117.0
0 125.6 125.6
0 125.6 125.1
1 134.8 135.3
2 145.1 145.6
3 155.7 157.2
4 168.0 168.1
5 180.5 180.6
6 197.7 198.6
7 211.4 209.7
8 224.0 222.7
9 238.5 233.7
10 250.9 245.8
11 261.5 256.4
12 274.0 270.4
13 291.3 283.6
14 304.2 296.5
15 317.9 309.5
And at this point the maximum power available at the oven-waist is 298mW. With 663mW available from the laser with a desired power setting of 700mW on the supply. Should make sure we understand where the power is being lost. The beam coming through the FI looks clean and unclipped, but there is some stray light around.
7 868.5 739.9
6 1324 1130
5 1765 1492
4 2214 1862
The plot looks pretty good, but again, there looks to be an offset on the 'fitted' curve. Taking a couple of additional points further on to make sure it all works out as the beam propagates. I took a few extra points at the suggestion of Kiwamu and Koji - see the zoomed out plot. The zoomed in plot has by-eye fit lines - again, because to get the right shape to fit the points there appears to be an offset. Where is that coming from? My suspicion is that the Beamscan doesn't take account of the any background zero offsets when calculating the 13.5% and we've been using low power when doing these measurements - very small focussed beams and didn't want to risk damage to the profiler head.
Decided to take a few measurements to test this theory. Trying different power settings and seeing if it gives different offset and/or a changed width size
7 984.9 824.0 very low power
7 931.9 730.3 low power
7 821.6 730.6 higher power
7 816.4 729.5 as high as I'm comfortable going
Trying this near the waist…
8.75 130.09 132.04 low power
8.75 106.58 105.46 higher power
8.75 102.44 103.20 as high as it can go without making it's saturated
So it looks like offset *is* significant and the Beamscan measurements are more accurate with more power to make the offsets less significant. Additionally, if this is the case then we can do a fit to the previous data (which was all taken with the same power setting) and simply allow the offset to be a free parameter without affecting the accuracy of the waist calculation. This fit and data coming to an e-log near you soon.
Of course, it looks from the plots above (well... the code that produces the plots above) that the waist is actually a little bit small (around 46um) so some adjustment of the last lens back along the beam by about half a cm or so might be required.
Skip to final thought ...
Kiwamu and I have set about measuring the contrast of the signal on the RF PD. We can only do this when the end green laser is locked to the cavity. This is because the green transmission through the cavity, when unlocked, is too low. Unfortunately, once we lock the green beam to the cavity, we can't keep the beatnote on the RF PD stable to within a few hundred Hz of DC (remember that the cavity is swinging around by a couple of FSRs). So we also lock the PSL to cavity.
At this point we're stuck because we can't get both of these beams resonant within the cavity AND have the frequency difference between them be less 1kHz - when the lasers are locked to the cavity, their frequencies are separated by an integer number of FSRs + a fixed frequency offset, f_offset, that is set by the phase difference on reflection from the coating between the two wavelengths (532nm and 1064nm). We can never get the frequency difference between the lasers to be less than this offset frequency AND still have them both locked to the cavity.
So our contrast measuring method will have to use the RF signal.
So this is our method. We know the incident power from each beam on the RF PD (see Kiwamu's elog entry here), but to recap,
P_green_PSL = 72 uW (as measured today)
P_green_XARM = 560 uW (as measured by Kiwamu last week).
The trans-impedance of the RF PD is 240 Ohms. We'll assume a responsitivity of 0.25 A/W. So, if the XARM transmission and PSL green beams are perfectly matched then the maximum value of the RF beat note should be:
RF_amplitude_max = 2* SQRT(P_green_PSL*P_green_XARM) * responsivity * transimpedance = 240*0.25*2*(72E-6*560E-6)^(1/2) (volts)
= 24 mV = -19.5 dBm (or 27.5dBm after the +47 dB from the two ZFL-1000LN+ amplifiers - with +15V in - that protrude from the top of the PD)
The maximum RF strength of the beat-note that we measure is around -75 dBm (at the RF output of the PD). This means the contrast is down nearly 600x from optimal. Or it means something is broken.
Final thought: at the end of this procedure we found that the RF beat note amplitude would jump to a different and much higher amplitude state. This renders a lot of the above useless until we discover the cause.
The attached table shows the amplitude of the green beat note when the end laser was in various states. We can increase the beat note amplitude dramatically by switching to a different states.
C1:GCX-GRN_REFL_DC: 638 counts
C1:GCV-XARM_BEAT_DC: (PSL blocked) 950 avg counts (zero = -794 counts)
amplitude of beat note: -23dBm (after PD + amps) (f ~ 30 MHz)?
C1:GCX-SLOW_SERVO2_OUT: 318 counts
C1:GCX-GRN_REFL_DC: 180 counts
C1:GCV-XARM_BEAT_DC: (PSL blocked) 1270 avg counts (zero = -794 counts)
C1:GCV-XARM_BEAT_DC: (PSL unblocked) 1700 avg counts (zero = -794 counts)
amplitude of beat note: -7dBm (after PD + amps) f = 60MHz
amplitude of beat note: 0dBm (after PD + amps) f = 30MHz
C1:GCX-SLOW_SERVO2_OUT: 290 counts
C1:GCX-GRN_REFL_DC: 220 counts
C1:GCV-XARM_BEAT_DC: (PSL blocked) 1120 avg counts (zero = -794 counts)
C1:GCV-XARM_BEAT_DC: (PSL unblocked) 1520 avg counts (zero = -794 counts)
amplitude of beat note: 0dBm (after PD + amps) f = 15MHz
C1:GCX-SLOW_SERVO2_OUT: 305 counts
PSL temp = ??
C1:PSL-FSS_SLOWM = -3.524
I went through the entries.
1. Give us a photo of the day. i.e. Faraday, tilted lens, etc...
2. After all, where did you put the faraday in the plot of the entry 4466?
3. Zoomed-in plot for the SHG crystal show no astigmatism. However, the zoomed out plot shows some astigmatism.
How consistent are they? ==> Interested in seeing the fit including the zoomed out measurements.
3. Zoomed-in plot for the SHG crystal show no astigmatism. However, the zoomed out plot shows some astigmatism.
How consistent are they? ==> Interested in seeing the fit including the zoomed out measurements.
OK. Taking these completely out of order in the easiest first...
2. The FI is between positions 27.75 and 32 on the bench - i.e. this is where the input and output apertures are. (corresponds to between 0.58 and 0.46 on the scale of those two plotsand just before both the vertical and horizontal waists) At these points the beam radius is around 400um and below, and the aperture of the Faraday is 4.8mm (diameter).
Laser set up - note the odd angles of the mirrors. This is where we're losing a goodly chunk of the light. If need be we could set it up with an extra mirror and send the light round a square to provide alignment control AND reduce optical power loss...
Faraday and angled lens - note that the lens angle is close to 45 degrees. In principle this could be replaced with an appropriate cylindrical lens, but as long as there's enough light passing through to the oven I think we're OK.
3. Fitting... coming soon once I work out what it's actually telling me. Though I hasten to point out that the latter points were taken with a different laser power setting and might well be larger than the actual beam width which would lead to astigmatic behaviour.
3. Zoomed-in plot for the SHG crystal show no astigmatism. However, the zoomed out plot shows some astigmatism.
How consistent are they? ==> Interested in seeing the fit including the zoomed out measurements.
Right. Fitting to the data. Zoomed out plots first. I used the general equation f(x) = w_o.*sqrt(1 + (((x-z_o)*1064e-9)./(pi*w_o.^2)).^2)+c for each fit which is basically just the Gaussian beam width parameter calculation but with an extra offset parameter 'c'
Vertical fit for zoomed out data:
Coefficients (with 95% confidence bounds):
c = 7.542e-06 (5.161e-06, 9.923e-06)
w_o = 3.831e-05 (3.797e-05, 3.866e-05)
z_o = 1.045 (1.045, 1.046)
Goodness of fit:
c = 1.083e-05 (9.701e-06, 1.195e-05)
w_o = 4.523e-05 (4.5e-05, 4.546e-05)
z_o = 1.046 (1.046, 1.046)
Adjusted R-square: 0.9998
OK. Looking at the plots and residuals for this, the deviation of the fit around the waist position, and in fact all over, looks to be of the order 10um. A bit large but is it real? Both w_o values are a bit lower than the 50um we'd like, but… let's check using only the zoomed in data - hopefully more consistent since it was all taken with the same power setting.
Vertical data fit using only the zoomed in data:
c = 1.023e-05 (9.487e-06, 1.098e-05)
w_o = 4.313e-05 (4.252e-05, 4.374e-05)
Horizontal data fit using only the zoomed in data:
c = 1.031e-05 (9.418e-06, 1.121e-05)
w_o = 4.41e-05 (4.332e-05, 4.489e-05)
The waists are both fairly similar this time 43.13um and 44.1um and the offsets are similar too - residuals are only spread by about 4um this time.
I'm inclined to trust the zoomed in measurement more due to the fact that all the data was obtained under the same conditions, but either way, the fitted waist is a bit smaller than the 50um we'd like to see. Think it's worthwhile moving the 62.9mm lens back along the bench by about 3/4 -> 1cm to increase the waist size.
I gutted one of the $2 red laser pointers to build a laser source whose amplitude we could modulate at RF frequencies. Basically, I cut off the bulk of the housing from the pointer and soldered a BNC connection into the two terminals that the 2x 1.5V batteries were connected to. When I applied 3V across this BNC connector the diode still worked. So far so good.
Next I added a bias tee to the input. I put 3V across the DC input of the bias tee and added a -3dBm signal into the RF port of the tee. The laser beam was incident on a PDA100A (bandwidth of 1.7MHz) and, sure enough, Kiwamu and I could see a flat response in the amplitude at a given drive frequency out to around 1.7MHz.
We should check the response on a faster PD to see how fast the laser diode is, but we should be able to use this now to check the RF response of the green beat note PD.
1. Add some capacitors across the DC input of the bias tee.
2. Do something about the switch on the laser diode.
3. Attach some labels to the laser that specify what is the required DC voltage and the maximum acceptable RF modulation amplitude.
I measured the DC response of the Green PD
Power into PD at DC (green laser pointer) = 285 uW
Voltage out of PD = 552mV/(100x SR560gain) = 5.52mV
Photocurrent = 5.52mV/(241 Ohms)*3 = 68.7uA
Responsivity = 68.7/285 = 0.24 A/W
Therefore, since the responsivity is in the correct range for a Silicon PD at 532nm, the DC output is giving us sensible response to an input signal.
But, there is a 2.12MHz, 328mV oscillation on the DC output irrespective of the incident power.
The doubling oven is now ready to go for the Y arm. The PPKTP crystal is mounted in the oven:
Note - the crystal isn't as badly misaligned as it looks in this photo. It's just an odd perspective shot. I then closed it up and checked to make sure the IR beam on the Y bench passes through the crystal. It does. Just need to tweak the waist size/position a bit and then we can actually double some frequencies!
I made a coarse noise budget in order to decide our next actions for the X arm green locking.
So be careful, this is not an accurate noise budget !
Some data are just coming from rough estimations and some data are not well calibrated.
Assuming all the noise are not so terribly off from the true values, the noise at high frequency is limited by the dark noise of the PD or it already reaches to the IR inloop signal.
The noise at low frequency is dominated by the intensity noise from the transmitted green light although we thought it has been eliminated by the comparator.
In any case I will gradually make this noise budget more accurate by collecting some data and calibrating them.
According to the plot what we should do are :
* More accurate PD noise measurement
* More accurate shot noise estimation
* Searching for a cause of the small beat signal (see here) because a bigger beat signal lowers the PD noise.
* Investigation of the Intensity noise
According to the measurement done by Aidan and me, there are two beat-note state.
One gave us a small beat signal and the other gave us a bigger signal by approximately 20 dB.
A possible reason for this phenomenon is that the end laser is operating at a special temperature that somehow drives the laser with two different modes at the same time.
So that it permits the laser sometimes locked with one of the two modes and sometimes with the other mode.
For the first step we will change the temperature such that the laser can run with a single stable mode.
Then for investigating it we will put a scanning cavity on the X end table to see if it really exhibits a two modes or not.
Last bit of oven matching for now.
I moved the lens before the oven position back along the beam path by about 1cm - waist should be just above position 9 in this case. Note - due to power-findings from previous time I'm maximising the power into the head to reduce the effect of offsets.
From position 9:
-1 121.1 123.6
0 112.5 113.8
1 106.4 106.1
2 102.9 103.4
3 103.6 103.6
4 106.6 107.4
5 111.8 112.5
6 118.2 120.1
7 126.3 128.8
8 134.4 137.1
9 143.8 146.5
10 152.8 156.1
11 163.8 167.1
12 175.1 176.4
13 186.5 187.0
14 197.1 198.4
15 210.3 208.9
16 223.5 218.7
17 237.3 231.0
18 250.2 243.9
19 262.8 255.4
20 274.7 269.0
21 290.4 282.3
22 304.3 295.5
23 316.7 303.1
Note - had to reduce power due to peak saturation at 15mm - don't think scale changed, but be aware just in case. And saturated again at 11. And again at 7. A little bit of power adjustment each time to make sure the Beamscan head wasn't saturating. Running the fit gives...
OK. The fit is reasonably good. Residuals around the area of interest (with one exception) are <+/- 2um and the waists are 47.5um (vertical) and 50.0um (horizontal) at a position of 9.09 on the bench. And the details of the fitting output are given below.
cf_(x) = w_o.*sqrt(1 + (((x-z_o)*1064e-9)./(pi*w_o.^2)).^2)+c
Coefficients (with 95% confidence bounds):
c = 5.137e-06 (4.578e-06, 5.696e-06)
w_o = 4.752e-05 (4.711e-05, 4.793e-05)
z_o = 1.04 (1.039, 1.04)
c = 3.81e-06 (2.452e-06, 5.168e-06)
w_o = 5.006e-05 (4.909e-05, 5.102e-05)
z_o = 1.04 (1.04, 1.04)
We now have green light at the Y end.
The set-up (with careful instructions from Kiwamu) - setting up with 100mW of IR into the oven.
Input IR power = 100mW measured.
Output green power = 0.11mW
(after using 2 IR mirrors to dump IR light before the power meter so losing a bit of green there light too)
And it's pretty circular-looking too. Think there might be a bit more efficiency to be gained near the edges of the crystal with internal reflections and suchlike things but that gives us an UGLY looking beam. Note - the polarisation is wrong for the crystal orientation so used a lambda/2 plate to get best green power out.
Efficiency is therefore 0.11/100 = 0.0011 (0.11%) at 100mW input power.
Temperature of the oven seems to be around 35.5degC for optimal conversion.
Took a picture. Ta-dah! Green light, and lots more where that came from! Well... about 3x more IR available anyway.
Since last Friday I have been testing the broadband RF photodetector in order to figure out the capability of S3399 with the similar circuit as Matt's BBPD
We also like to figure out if it has sufficient performance for the 40m green locking.
The circuit diagram is shown in the first attachment. The RF amplifier is attached at the diode while the reverse bias voltage is applied at the other side of the diode. The amplifier's input impedance is used as the transimpedance resister. Note that the bandwidth of this configuration is limited by the RC filter that consists of the junction capacitance of the diode, the series resistance of the diode, and the transimpedance resister. This cut off freq is in general lower than that cut off obtained with the usual transimpedance amplifier which has the readout resister at the feedback path of the opamp.
The transfer function of the PD is measured using Jenne's laser. At the reverse bias voltage of 30V, the -3dB bandwidth of 178MHz was obtained. This is quite high bandwidth for the most of the applications at the 40m.
Because of the low transimpedance the low-noise level of the RF amplifier is very crucial. Recently we can obtain an ultra low noise RF amplifier like Teledyne Cougar AC688 which has the NF of 0.9dB with the bandwidth between 10MHz - 600MHz. Next step will be to obtain this kind of amplifier to test the noise performance.
Last Thursday, I tested Newport Servo Controller LB1005 with the X_arm green PDH servo.
The setup and the settings I could lock the arm is depicted in the attached figure.
To lock the cavity, follow the steps below
1) Toggle the switch to the "lower" position. This disengages the servo and reset the integrator.
2) Toggle the switch to the "middle" position. The zero freq is set to the "PI corner" freq. At the low freq the gain is limited
at the value of "LF Gain Limit". This gives us a single pole at the low freq.
3) Once the lock is acquired, toggle the switch to the "upper" position. This moves the pole freq to DC, resulting in the complete integration of the signal at the low frequency.
I measured the openloop transfer function (attachment 2). The amp is quite fast and exhibits almost no phase delay upto 100kHz.
The UGF was 10kHz with the phase mergin of ~45deg. I had to tune the input offset carefully to stay at the center of the resonance.
It turned out that the dark noise from the beat PD and the shot noise on the beat PD was overestimated.
So I corrected them in the plot of the last noise budget (#4482).
Additionally I added the end laser error signal in the plot. Here is the latest plot.
The end laser error spectrum is big enough to cover most of the frequency range.
(although it was taken at a different time from the other curves.)
I moved the Hartmut Green PD to the Jenne laser bench to try to determine if the response at RF was reasonable or somehow very much smaller than it should be. It was set up as shown in the attached diagram. The first pass at this was by comparing the ratio of the RF photocurrent of the green PD to the RF photocurrent of the New Focus 1611 InGaAs PD. That ratio (at a sufficiently low frequency) should be the same as the ratio the DC photocurrents of the two PDs.
Using the network analyzer I measured the ratio of the voltages of the two RF signals (and then scaled each of these by the respective transimpedances of the PDs: 700 Ohms for the 1611 and 240 Ohms for the Harmut PD). The resulting ratio is shown in the attached plot.
I measured the DC voltages from each PD and scaled those by the transimpedances to get the photocurrent (10 kOhm for the 1611 and 80 Ohm effective for the Harmut PD). The ratio of the DC photocurrents was 0.37. This is roughly 3x the ratio of the RF photocurrents at 500kHz (=0.14). This discrepancy is uncomfortably large.
The full set of measurements is given in the table below:
There is one other troubling point: using the estimate of responsivity on the Harmut PD * incident power * transimpedance at DC = (0.02A/W) * (0.28mW) * (80 V/A) = 0.45 mV.
But the measured DC voltage is 6.5mV = inconsistent.
Every so often things just work out. You do the calculations, you put the lenses on the bench, you manually adjust the pointing and fiddle with the lenses a bit, you get massive chunks of assistance from Kiwamu to get the alignment controls and monitors set up and after quite a bit of fiddling and tweaking the cavity mirror alignment you might get some nice TEM_00 -like shapes showing up on your Y-arm video monitors.
So. We have resonating green light in the Y-arm. The beam is horribly off-axis and the mode-matching, while close enough to give decent looking spots, has in no way been optimised yet. Things to do tomorrow - fix the off-cavity-axis problem and tweak up the mode-matching... then start looking at the locking...
I think I had underestimated the responsivity of the Silicon PD at 1064nm. The previous value was based on a rough search online for the responsivity of Silicon (I couldn't find the product number of the actual PD we are using). For instance, the PDA100A Si detector from Thorlabs has a responsivity of 0.35-0.4A/W at 1064nm.
If we calculate the responsivity of the Hartmut PD from the measurements I made today (input power = 0.300mW, output voltage = 5.56mV, effective transimpedance = 80 Ohms), then the responsivity at 1064nm is 0.23 A/W which is not an unreasonable number given the response of the Thorlabs detector.
Responsivity of SGD-444A
For instance, the PDA100A Si detector from Thorlabs has a responsivity of 0.35-0.4A/W at 1064nm.
Having convinced myself that the green Hartmut PD is giving an acceptable response at RF frequencies I decided to double-check the beatnote at IR (fiber transmission from the X-end beating with the PSL). This took a while because I had to realign the beam into the fiber at the X-end (I had a PD monitoring the output from the fiber on the PSL table and 40m of BNC cable giving me the signal from it at the X-end).
Eventually, I managed to get a beatnote on the PD. At first there was no signal at the temperature calculated using Koji and Suresh's calibration, but it turned out that the mode-overlap wasn't good enough on the PD. Now I can clearly see beats between a couple of modes, one of which is much stronger than the other. I think we should use a frequency discriminator on the output from the IR PD to servo the end laser and keep the strong beat note within <100MHz of DC.
I started to modify another green PD set.
It so far has the transimpedance of 240 Ohm on CLC409 for the RF output.
It shows the BB output upto ~100MHz.
The measurement shows the transimpedenca of ~90Ohm which is ~25% smaller than the expected gain of 120Ohm.
It is calibrated based on the transimpedances of Newfocus 1611 (10kOhm and 700Ohm for AF and RF).
The next step is to change the transimpedance resister to 2k and replace the PD to S3399 Si PD, which has the diameter of 3mm.
Then, the noise level will be measured. (and replace the RF opamp if necessary)
The Y-arm can now be locked with green light using the universal PDH servo. Modulation frequency is now 277kHz - chosen because it seems to produce smaller offsets due to AM effects
To lock, turn on the servo, align the system to give nice circular-looking TEM_00 resonances, and wait for a good one. It'll lock on a decent mode for a few seconds and then you can turn on the local boost and watch it lock for minutes and minutes and minutes.
The suspensions are bouncing around a bit on the Y-arm and the spot is quite low on the ETMY and a little low on ITMY, but from this point it can be tweaked and optimised.
OK… the Y-arm may be locked with green light, which was the goal, and this is all good but it's not yet awesome. Awesome would be locked and aligned properly and quiet and optimised. So... in order to assist in increasing the awesome-osity, here are a few stream-of-consciousness thoughts and stuff I've noticed and haven't had time to fix/investigate or have otherwise had pointed out to me that may help...
Firstly, the beam is not aligned down the centre of the cavity. It's pretty good horizontally, but vertically it's too low by about 3/4->1cm on ETMY. The mirrors steering the beam into the cavity have no more vertical range left, so in order to get the beam higher the final two mirrors will have to be adjusted on the bench. Adding another mirror to create a square will give more range AND there will be less light lost due to off 45degree incident angles. When I tried this before I couldn't get the beam to return through the Faraday, but now the cavity is properly aligned this should not be a problem.
A side note on alignment - while setting cameras and viewports and things up, Steve noticed that one of the cables to one of the coils (UL) passes behind the ETMY. One of the biggest problems in getting the beam into the system to begin with was missing this cable. It doesn't fall directly into the beam path if the beam is well aligned to the cavity, but for initial alignment it obscures the beam - this may be a problem later for IR alignment.
Next, the final lambda/2 waveplate is not yet in the beam. This will only become a problem when it comes to beating the beams together at the vertex, but it WILL be a problem. Remember to put it in before trying to extract signals for full LSC cavity locking.
Speaking of components and suchlike things, the equipment for the green work was originally stored in 3 plastic boxes which were stored near the end of the X-arm. These boxes, minus the components now used to set up the Y-end, are now similarly stored near the end of the Y-arm.
Mechanical shutter - one needs to be installed on the Y-end just like the X-end. Wasn't necessary for initial locking, but necessary for remote control of the green light on/off.
Other control… the Universal PDH box isn't hooked up to the computers. Connections and such should be identical to the X-arm set-up, but someone who knows what they're doing should hook things up appropriately.
More control - haven't had a chance to optimise the locking and stability so the locking loop, while it appears to be fairly robust, isn't as quiet as we would like. There appears to be more AM coupling than we initially thought based on the Lightwave AM/PM measurements from before. It took a bit of fiddling with the modulation frequency to find a quiet point where the apparent AM effects don't prevent locking. 279kHz is the best point I've found so far. There is still a DC offset component in the feedback that prevents the gain being turned up - unity gain appears limited to about 1kHz maximum. Not sure whether this is due to an offset in the demod signal or from something in the electronics and haven't had time left to check it out properly yet. Again, be aware this may come back to bite you later.
Follow the bouncing spot - the Y-arm suspensions haven't been optimised for damping. I did a little bit of fiddling, but it definitely needs more work. I've roughly aligned the ETMY oplev since that seems to be the mass that's bouncing about most but a bit of work might not go amiss before trusting it to damp anything.
Think that's about all that springs to mind for now…
Thanks to everyone at the 40m lab for helping at various times and answering daft questions, like "Where do you keep your screwdrivers?" or "If I were a spectrum analyser, where would I be?" - it's been most enjoyable!
Y-end PDH electronics.
The transfer function of the Y-end universal PDH box:
I was investigating the beat note amplitude on the vertex PD again yesterday. The incident power on the PD was 150uW in the PSL green beam and 700uW in the X-ARM green beam. With perfect overlap and a transimpedance of 240, I expected to get a beat note signal of around 25mV or -19dBm. Instead, the size was -57dBm. Bryan and I adjusted the alignment of the green PSL beam to try and improve the mode overlap but we couldn't do much better than about -50dBm. (The noise floor of the PD is around -65dBm).
When we projected the beams to the wall of the enclosure, the xarm beam was 2 to 3x as large as the PSL green beam, indicating that the beam size and/or curvatures on the PD were less than ideal. There is a telescope that the XARM beam goes through just before it gets to the PD. I mounted the second lens in this telescope on a longitudinal translation stage. With some finagling of the position of that lens we were able to improve the beatnote signal strength to -41dBm.
Obviously the ideal solution would be to measure the beam size and RoC of the PSL beam and XARM beams and then design a telescope that would match them as precisely as possible because there's still another 20dB signal strength to be gained.
With some assistance from Kiwamu and Koji, I've drawn up the electronics design for the Beat Box for the vertex green locking. The Omingraffle schematic is posted on the Green Locking Wiki page. It's also attached below. Some final touches are necessary before we can Altium this up.
Attachment 1: Schematic of beatbox
Attachment 2: Front and back panel designs.
- AC coupling for the comparator circuit of the green locking
In order to relieve the power consumption of the RF buffer, ac coupling circuits have been added.
The ac coupling before the buffer amp helps to relieve the power consumption in the chip.
But because of the distortion of the signal (and the limitation of the bandwidth), the output still has some DC (~0.6V).
Therefore, the output is also AC coupled.
Note that the BW pin of BUF634P should be directly connected to -15V in order to keep the bandwidth of the buffer.
The drawings are also uploaded on the green electronics wiki
1.The aim is the laser frequency stabilisation of PSL and AUX.
2.As a first step we want to couple some of the AUX laser beam into a single mode optical fibre and route the fibre to the PSL table.
3.The position of the optical fibre on the ETMY table is shown by the coupler in the attached picture. The yellow lines show the new scheme we want to implement.
4.WHAT WE DID TODAY.
The Lightwave NPRO power supply which is being shared between the AS table and the ETMY table has been shifted back to the ETMY table.
The current to the laser is set at 1.5A. The laser output is 200mW at this current level.